Re: [PATCH v2 3/3] sched: increase SCHED_LOAD_SCALE resolution

[Ingo Molnar]

* Nikhil Rao <ncrao@xxxxxxxxxx> wrote:

> On Wed, May 18, 2011 at 1:26 PM, Ingo Molnar <mingo@xxxxxxx> wrote:
> >
> > * Nikhil Rao <ncrao@xxxxxxxxxx> wrote:
> >
> >> +#if BITS_PER_LONG > 32
> >> +# define SCHED_LOAD_RESOLUTION       10
> >> +# define scale_load(w)               (w << SCHED_LOAD_RESOLUTION)
> >> +# define scale_load_down(w)  (w >> SCHED_LOAD_RESOLUTION)
> >> +#else
> >> +# define SCHED_LOAD_RESOLUTION       0
> >> +# define scale_load(w)               (w)
> >> +# define scale_load_down(w)  (w)
> >> +#endif
> >
> > We want (w) in the other definitions as well, to protect potential operators
> > with lower precedence than <<. (Roughly half of the C operators are such so
> > it's a real issue, should anyone use these macros with operators.)
> >
> >> +#define SCHED_LOAD_SHIFT     (10 + (SCHED_LOAD_RESOLUTION))
> >
> > that () is not needed actually, if you look at the definition of
> > SCHED_LOAD_RESOLUTION.
> >
> > So you could move the superfluous () from here up to the two definitions above
> > and thus no parentheses would be hurt during the making of this patch.
> >
> 
> Ah, thanks for the explanation. Does this look OK?
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index f2f4402..c34a718 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -788,9 +788,28 @@ enum cpu_idle_type {
>  };
> 
>  /*
> - * Increase resolution of nice-level calculations:
> + * Increase resolution of nice-level calculations for 64-bit architectures.
> + * The extra resolution improves shares distribution and load balancing of
> + * low-weight task groups (eg. nice +19 on an autogroup), deeper taskgroup
> + * hierarchies, especially on larger systems. This is not a user-visible change
> + * and does not change the user-interface for setting shares/weights.
> + *
> + * We increase resolution only if we have enough bits to allow this increased
> + * resolution (i.e. BITS_PER_LONG > 32). The costs for increasing resolution
> + * when BITS_PER_LONG <= 32 are pretty high and the returns do not justify the
> + * increased costs.
>   */
> -#define SCHED_LOAD_SHIFT       10
> +#if BITS_PER_LONG > 32
> +# define SCHED_LOAD_RESOLUTION 10
> +# define scale_load(w)         ((w) << SCHED_LOAD_RESOLUTION)
> +# define scale_load_down(w)    ((w) >> SCHED_LOAD_RESOLUTION)
> +#else
> +# define SCHED_LOAD_RESOLUTION 0
> +# define scale_load(w)         (w)
> +# define scale_load_down(w)    (w)
> +#endif
> +
> +#define SCHED_LOAD_SHIFT       (10 + SCHED_LOAD_RESOLUTION)
>  #define SCHED_LOAD_SCALE       (1L << SCHED_LOAD_SHIFT)
> 
>  /*
> 
> 
> >> +             if (BITS_PER_LONG > 32 && unlikely(w >= WMULT_CONST))
> >>                       lw->inv_weight = 1;
> >> +             else if (unlikely(!w))
> >> +                     lw->inv_weight = WMULT_CONST;
> >>               else
> >> +                     lw->inv_weight = WMULT_CONST / w;
> >
> > Ok, i just noticed that you made use of BITS_PER_LONG here too.
> >
> > It's better to put that into a helper define, something like
> > SCHED_LOAD_HIGHRES, which could thus be used like this:
> >
> >                if (SCHED_LOAD_HIGHRES && unlikely(w >= WMULT_CONST))
> >
> > then, should anyone want to tweak the condition for SCHED_LOAD_HIGHRES, it
> > could be done in a single place. It would also self-document.
> >
> 
> Hmm, that particular use of BITS_PER_LONG was not touched by this
> patch. This patch only changes lw->weight to use the local variable w.
> The (BITS_PER_LONG & > WMULT_CONST) check is required on 64-bit
> systems irrespective of the load-resolution changes.

my bad, i confused it with the resolution changes.

It's fine as-is then i guess. Mind reposting the full patch again, with all 
updates included and the subject line changed to make it easy to find this 
patch in the discussion?

Thanks,

	Ingo
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